This application relates to lenses which may be used in lamps, particularly automotive headlamps, which provide a shift in chromaticity of the light source beam.
Automotive headlamps are highly controlled products that must meet the SAE performance standard (SAE J1383) to be commercialized. To be compliant, the combination bulb (i.e. the light source)/lens must emit a “white” color and provide enough light output (usually characterized by the total luminous flux “isocandela” and “maximum candela” point intensity testing) in a homogeneous manner. Specifications have been defined around the white beam color as presented in the SAE J578 standard. The white beam color is defined as a small portion of the color space in the CIE 1931 chromaticity diagram. The allowed portion of the color space if defined by blue, yellow, green, purple, and red boundaries that stem from the CIE 1931 x and y color coordinates. Commercially available headlamps use different types of bulbs but usually a “natural” colored lens or slightly tinted lens. In general, these lenses have a clear appearance but could display a very subtle blue or yellow tint. The most common bulb on the market is a halogen bulb. In the past few years, high performance bulbs have been introduced. These new bulbs usually referred to as HID (“High Intensity Discharge”) are in fact Xenon lamps. It is well known to those skilled in the art that the power spectral distribution of a Xenon bulb is different from a halogen bulb. For example, a Xenon bulb will emit more energy at lower wavelengths and especially in the 300 to 500 nm range that corresponds to the long UV up to violet/blue-green. As a result, the light emitted from the HID is bluer compared to a halogen bulb which will consequently appear more yellow. When mounted in a headlamp, the beam emitted from a HID/“natural” lens combination will appear whiter. A whiter beam is commonly acknowledged as more efficient since it enhances the road visibility at night. However, there are two major disadvantages to the use of HID bulbs in headlamps. Firstly, these high performance bulbs are extremely expensive compared to halogen bulbs. As a result, headlamps based on HID bulbs are a limited market, often offered as an option on vehicles for an extra-cost in the range of $300 to $800 per unit. Secondly, recent studies have shown that these headlamps have a tendency to cause more discomfort glare for oncoming drivers.
Automotive headlamp lenses are usually made of natural color or slightly tinted polycarbonate as a main material. The primary reasons behind the use of polycarbonate are its relatively high glass transition temperature, impact resistance and excellent clarity/light transmission in the visible range. Lexan® LS-2 polycarbonate is one of the leading materials currently in use for automotive lenses; including headlamp lenses, bezels and taillight lenses. Other high glass transition temperature materials are also being used including copolymers but their natural color or light transmission sometimes renders the emitted headlamp beam of a lesser quality. It is well known to those skilled in the art of coloring automotive lenses that the natural or slightly tinted polycarbonate lenses are obtained by addition of a small amount of organic colorants (i.e. dyes or pigments). For example, a blue dye is added to a yellow formulation to neutralize the color (i.e. make the polycarbonate more colorless or “natural”). The main downside of coloring is the decrease in light transmission that results from the absorption of the colorants even when they are present in the polymer matrix at a ppm loading or below. Consequently, the great majority of the lenses that are mounted in headlamps are “natural” or barely tinted.